The present invention relates to a device for controlling refrigeration cycle capacity and, more particularly, to a control device for ensuring the reliable operation of a compressor.
Referring first to FIG. 5, the construction of an air conditioner of the type in which the rotation (r.p.m.) of a compressor is varied by means of an inverter, thereby varying the refrigeration cycle capacity, will be described. In FIG. 5, reference numeral 1 designates a compressor; 2, an indoor heat exchanger which functions as a condensor in case of heating; and 3, an outdoor heat exchanger which functions as an evaporator. The indoor and outdoor heat exchangers 2 and 3 are intercommunicated through capillary tubes 4a and 4b which constitute a decompression device and a bypass circuit 6 with a check valve 5 which is used to bypass the capillary tube 4a in case of cooling operation. Reference numeral 7 designates a four port connection valve for selecting a refrigeration line. The rotation of the compressor 1 is controlled by an inverter control device 8. The output signal from a temperature sensor 9 which detects a suction temperature of the indoor heat exchanger 2 is applied to an arithmetic unit 10 comprising a microcomputer or the like and in response to a temperature difference between the detected suction temperature and a set point temperature, the output signal from the arithmetic circuit 10 is applied through a driving circuit 11 to an inverter 12, thereby controlling the rotation of the compressor 1. Reference numeral 13 is a commercial power source which is electrically connected not only to the arithmetic circuit 10 through a power supply 14 but also to the inverter 12 through a converter 15.
In the air conditioner with the above-described construction, the number of revolutions of the compressor 1 is dependent upon the difference between an indoor temperature and a set point. As a result, in case of the compressor 1 of the type in which, as in the case of a rotary compressor, a reservoir at which is stored a lubricating oil has a high pressure, the compressor 1 is driven at a low temperature and at a high pressure and a large amount of a lubricating oil is dissolved into a refrigerant and fed into a circulating refrigerant, resulting in the decrease of the lubricating oil for the compressor. When the compressor is rotated at a high rotational velocity for a relatively long period of time with an insufficient quantity of a lubricating oil which has been caused by the abovementioned drive of the compressor 1, seizure and wear of sliding surfaces tend to occur very frequently, causing the problem of reliable operation of the air conditioner.